As integrated circuit (IC) dimensions shrink, the ability to deposit conformal thin film layers with excellent step coverage at low deposition temperatures is becoming increasingly important. Thin film layers are used, for example, as MOSFET gate dielectrics, DRAM capacitor dielectrics, adhesion promoting layers, diffusion barrier layers, and seed layers for subsequent deposition steps. Low temperature processing is desired, for example, to prevent unwanted diffusion of shallow junctions, to better control certain reactions, and to prevent degradation of previously deposited materials and their interfaces.
The need for conformal thin film layers with excellent step coverage is especially important for high aspect ratio trenches and vias, such as those used in metallization layers of semiconductor chips. For example, copper interconnect technology requires a continuous thin film barrier layer and a continuous thin film copper seed layer to coat the surfaces of trenches and vias patterned in an insulating dielectric prior to filling the features with copper by electrochemical deposition (ECD or electroplating).
A highly conformal, continuous barrier layer is required to prevent copper diffusion into the adjacent semiconductor (i.e., silicon) material or dielectric. The barrier layer also often acts as an adhesion layer to promote adhesion between the dielectric and the copper seed layer. Low dielectric constant (i.e., low-k) dielectrics are typically used to reduce inter- and intra-line capacitance and cross-talk, but often suffer from poorer adhesion and lower thermal stability than traditional oxide dielectrics, making the choice of a suitable adhesion layer more critical. A non-conformal barrier layer, or one with poor step coverage or discontinuous step coverage, can lead to copper diffusion and current leakage between adjacent metal lines or to delamination at either the barrier-to-dielectric or barrier-to-seed layer interfaces, both of which adversely affect product lifetime and performance. The barrier layer should also be uniformly thin, to most accurately transfer the underlying trench and via sidewall profile to the subsequent seed layer, and have a low film resistivity (e.g., ρ<500 μΩ-cm) to lessen its impact on the overall conductance of the copper interconnect structures.
A highly conformal, uniformly thin, continuous seed layer with low defect density is required to prevent void formation in the copper wires. The seed layer carries the plating current and acts as a nucleation layer. Voids can form from discontinuities or other defects in the seed layer, or they can form from pinch-off due to gross overhang of the seed layer at the top of features, both trenches and vias. Voids adversely impact the resistance, electromigration, and reliability of the copper lines, which ultimately affects the product lifetime and performance.
Traditional thin film deposition techniques, for example, physical vapor deposition (PVD) and chemical vapor deposition (CVD), are increasingly unable to meet the requirements of advanced thin films. PVD, such as sputtering, has been used for depositing conductive thin films at low cost and at relatively low substrate temperature. Unfortunately, PVD is inherently a line of sight process, resulting in poor step coverage in high aspect ratio trenches and vias. Advances in PVD technology to address this issue have resulted in high cost, complexity, and reliability issues. CVD processes can be tailored to provide conformal films with improved step coverage. Unfortunately, CVD processes often require high processing temperatures, result in the incorporation of high impurity concentrations, and have poor precursor (or reactant) utilization efficiency, leading to a high cost of ownership.
Atomic layer deposition (ALD), or atomic layer chemical vapor deposition (AL-CVD), is an alternative to traditional CVD methods to deposit very thin films. ALD has several advantages over PVD and traditional CVD. ALD can be performed at comparatively lower temperatures (which is compatible with the industry's trend toward lower temperatures), has high precursor utilization efficiency, can produce conformal thin film layers (i.e., 100% step coverage is theoretically possible), can control film thickness on an atomic scale, and can be used to “nano-engineer” complex thin films.
A typical ALD process differs significantly from traditional CVD processes. In a typical CVD process, two or more reactant gases are mixed together in the deposition chamber where either they react in the gas phase and deposit on the substrate surface, or they react on the substrate surface directly. Deposition by CVD occurs for a specified length of time, based on the desired thickness of the deposited film. Since this specified time is a function of the flux of reactants into the chamber, the required time may vary from chamber to chamber.
In a typical ALD process deposition cycle, each reactant gas is introduced sequentially into the chamber, so that no gas phase intermixing occurs. A monolayer of a first reactant is physi- or chemisorbed onto the substrate surface. Excess first reactant is pumped out, possibly with the aid of an inert purge gas. A second reactant is introduced to the deposition chamber and reacts with the first reactant to form a monolayer of the desired thin film via a self-limiting surface reaction. The self-limiting reaction halts once the initially adsorbed first reactant fully reacts with the second reactant. Excess second reactant is pumped out, again possibly with the aid of an inert purge gas. A desired film thickness is obtained by repeating the deposition cycle as necessary. The film thickness can be controlled to atomic layer (i.e., angstrom scale) accuracy by simply counting the number of deposition cycles.
Physisorbed precursors are only weakly attached to the substrate. Chemisorption results in a stronger, more desirable bond. Chemisorption occurs when adsorbed precursor molecules chemically react with active surface sites. Generally, chemisorption involves cleaving a weakly bonded ligand (a portion of the precursor) from the precursor, leaving an unsatisfied bond available for reaction with an active surface site.
The substrate material can influence chemisorption. In current dual damascene copper interconnect structures, a barrier layer such as tantalum (Ta) or tantalum nitride (TaN) must often simultaneously cover silicon dioxide (SiO2), low-k dielectrics, nitride etch stops, and any underlying metals such as copper. Materials often exhibit different chemical behavior, especially oxides versus metals. In addition, surface cleanliness is important for proper chemisorption, since impurities can occupy surface bonding sites. Incomplete chemisorption can lead to porous films, incomplete step coverage, poor adhesion between the deposited films and the underlying substrate, and low film density.
The ALD process temperature must be selected carefully so that the first reactant is sufficiently adsorbed (e.g., chemisorbed) on the substrate surface, and the deposition reaction occurs with adequate growth rate and film purity. A temperature that is too high can result in desorption or decomposition (causing impurity incorporation) of the first reactant. A temperature that is too low may result in incomplete chemisorption of the first precursor, a slow or incomplete deposition reaction, no deposition reaction, or poor film quality (e.g., high resistivity, low density, poor adhesion, and/or high impurity content).
Traditional ALD processes have several disadvantages. First, since the process is entirely thermal, selection of an appropriate process temperature is often confined to a narrow temperature window. Second, the small temperature window limits the selection of available precursors. Third, metal precursors that fit the temperature window are often halides (e.g., compounds that include chlorine, flourine, or bromine), which are corrosive and can create reliability issues in metal interconnects. Fourth, either gaseous hydrogen (H2) or elemental zinc (Zn) is often used as the second reactant to act as a reducing agent to bring a metal compound in the first reactant to the desired oxidation state of the final film. Unfortunately, H2 is an inefficient reducing agent due to its chemical stability, and Zn has a low volatility and is generally incompatible with IC manufacturing. Thus, although conventional ALD reactors are suitable for elevated-temperature ALD, they limit the advancement of ALD processing technology.
Plasma-enhanced ALD, also called radical enhanced atomic layer deposition (REALD), was proposed to address the temperature limitations of traditional thermal ALD. For example, in U.S. Pat. No. 5,916,365, the second reactant passes through a radio frequency (RF) glow discharge, or plasma, to dissociate the second reactant and to form reactive radical species to drive deposition reactions at lower process temperatures. More information on plasma-enhanced ALD is included in “Plasma-enhanced atomic layer deposition of Ta and Ti for interconnect diffusion barriers,” by S. M. Rossnagel, et al., Journal of Vacuum Science and Technology B 18(4) July/August 2000 pp. 2016-2020.
Plasma enhanced ALD, however, still has several disadvantages. First, it remains a thermal process similar to traditional ALD since the substrate temperature provides the required activation energy, and therefore the primary control, for the deposition reaction. Second, although processing at lower temperatures is feasible, higher temperatures must still be used to generate reasonable growth rates for acceptable throughput. Such temperatures are still too high for some films of interest in IC manufacturing, particularly polymer-based low-k dielectrics that are stable up to temperatures of only 200° C. or less. Third, metal precursors, particularly for tantalum (Ta), often still contain chlorine as well as oxygen impurities, which results in low density or porous films with poor barrier behavior and chemical instability. Fourth, the plasma enhanced ALD process, like the conventional sequential ALD process described above, is fundamentally slow since it includes at least two reactant gases and at least two purge or evacuation steps, which can take up to several minutes with conventional valve and chamber technology.
Conventional ALD reactors, including plasma enhanced ALD reactors, include a vertically-translatable pedestal to achieve a small process volume, which is important for ALD. A small volume is more easily and quickly evacuated (e.g., of excess reactants) than a large volume, enabling fast switching of process gases. Also, less precursor is needed for complete chemisorption during deposition. For example, the reactors of U.S. Pat. No. 6,174,377 and European Patent No. 1,052,309 A2 feature a reduced process volume located above a larger substrate transfer volume. In practice, a typical transfer sequence includes transporting a substrate into the transfer volume and placing it on top of a moveable pedestal. The pedestal is then elevated vertically to form the bottom of the process volume and thereby move the substrate into the process volume. Thus, the moveable pedestal has at least a vertical translational and possibly a second rotational degree of freedom (for high temperature process uniformity).
Typical ALD reactors have significant disadvantages. First, conventional ALD reactors suffer from complex pedestal requirements, since the numerous facilities (e.g., heater power lines, temperature monitor lines, and coolant channels) must be connected to and housed within a pedestal that moves. Second, in the case of plasma enhanced ALD, the efficiency of radical delivery for deposition of conductive thin films is significantly decreased in downstream configurations in which the radical generating plasma is contained in a separate vessel remote from the main process chamber (see U.S. Pat. No. 5,916,365). Both gas phase and wall recombinations reduce the flux of useful radicals to the substrate. In the case of atomic hydrogen (H), recombination results in diatomic H2, a far less effective reducing agent. Other disadvantages of known ALD reactors exist.
Accordingly, improved ALD reactors are desirable to make ALD better suited for commercial IC manufacturing. Desirable characteristics of such reactors might include higher throughput, improved deposited film characteristics, better temperature control for narrow process temperature windows, and wider processing windows (e.g., in particular with respect to process temperature and reactant species).